Thermoelectric generator devices convert heat energy from a heat source directly to electrical energy. In one type of thermoelectric generator, the electrical energy is generated by electrochemically expanding alkali metal across a solid electrolyte. Such generators, wherein sodium metal is employed as the working substance, have been described in U.S. Pat. Nos. 3,458,356 and 4,510,210, and are commonly referred to as "sodium heat engines". This type of thermoelectric generator is discussed herein as exemplary of one type of generator in which the article of this invention may be suitably used.
The sodium heat engine generally comprises a closed container separated into a first and second reaction zone by a solid electrolyte. Liquid sodium metal is present in the first reaction zone (i.e., on one side of the solid electrolyte). In the second reaction zone (i.e., on the other side of the solid electrolyte), a permeable, electrically conducting electrode is in contact with the solid electrolyte. During operation of such a device, a heat source raises the temperature of liquid sodium metal within the first reaction zone to a high temperature and corresponding high vapor pressure, which creates a sodium vapor pressure difference across the solid electrolyte. In response to this pressure difference, the elemental sodium gives up electrons to the electrode in contact with the sodium metal and the resulting sodium ions migrate through the solid electrolyte. The electrons, having passed through an external load, neutralize sodium cations at the permeable electrode-solid electrolyte interface. Elemental sodium metal evaporates from the permeable electrode and migrates through the low pressure second reaction zone to a low temperature condenser. The condensed liquid sodium may then be returned back to the higher temperature first reaction zone.
In the thermoelectric generator system just described, the electrode on the surface of the electrolyte from which the alkali metal ions emerge is a positive electrode and must be present in order to transfer electronic charge from the external circuit to the alkali metal ions. This completes the electrochemical circuit required for operation of the generator. The operation of such thermoelectric generator systems require electrodes possessing special properties, some of which are difficult to optimize simultaneously. For example, it is necessary for efficient generator operation that the positive electrode conduct electrons from the electrical load to a broad surface of the electrolyte, doing so with low electrical resistance. At the same time, it is also necessary for the positive electrode to permit the passage of alkali metal atoms from the electrolyte-electrode interface through the electrode to the opposite electrode surface, from which they may pass to the condenser. While the former requirement is more likely to be attained by dense, thick electrodes to promote low resistance, the latter requirement suggests thin, permeable electrodes to promote the easy passage of the alkali metal through the electrode. Additionally, the electrodes must be relatively unreactive with the alkali metal and have low vapor pressure to prevent their loss through evaporation in the high temperature, high vacuum environment in which they operate. Still further, the electrode material must have a thermal expansion coefficient offering a fair match to that of the electrolyte substance. This is necessary in order to prevent delamination of the electrode from the electrolyte which could result from differential expansion and contraction of the electrode and electrolyte materials during the heating and cooling cycles to which such systems are exposed during use.
U.S. Pat. No. 4,049,877, to Saillant et al, is directed to a thermoelectric generator wherein the improvement comprises employing, as the electrode, a porous metal film deposited on the solid electrolyte by chemical vapor deposition. Among the metals taught as suitable for use as the electrode are molybdenum, tungsten, chromium, nickel and iron. Cole, in U.S. Pat. No. 4,175,164, teaches that the surface configuration of electrodes formed by chemical vapor deposition techniques (such as those in the above Saillant et al patent) may be modified by exposing such electrodes to oxidizing conditions, followed by reducing conditions. It is taught by Cole that these conditions effect an oxidation, reduction and consequent redeposition of the already deposited electrode which makes it desirably more porous, thus providing improved electrode efficiency. Both of these patents are commonly assigned with this application. However, these metal electrodes, while being capable of excellent power and efficiency in the initial stages of operation, have shown a tendency to lose power with operating time at high temperatures. It is believed that in the presence of oxygen and sodium, the refractory metal can form compounds which exhibit liquid phases at the temperatures of generator operation. Such liquid phases provide both a conducting path for sodium and good physical contact between electrode and electrolyte, resulting in the excellent initial power. Subsequent loss of these liquid phases through evaporation or decomposition leads to the power decrease observed.